Related rates of increase

a spherical balloon is to be filled with water so that there is a constant increase in the rate of its surface area of 3cm2/sec .
a) Find the rate of increase in the radius when the radius is 3cm.
b) Find the volume when the volume is increasing at a rate of 10cm3=sec.

A=surface area, V=volume
a) i thought was fairly easy and got 1/(8Pi) cm/sec
b) was a bit more trickier here is what i did